This invention relates to aluminum and aluminum base alloy articles having on the surface thereof a high brightness, corrosion resistant finish, and the method for their manufacture. More particularly, the invention relates to aluminum base alloy automotive vehicle bumpers having a synthetic resin lacquer coating on their surface and exhibiting a high degree of spectral reflectivity.
Automotive vehicle bumpers and other wrought articles of trim on automobiles, such as hub caps, grill work, and the like, have been made predominantly of chromium plated steel. Owing to the susceptibility of the chromium plated surfaces to the deleterious effects of weather, salt spray, and atmospheric corrosion, it has been conventional practice to apply to said surfaces a protective film or coating of a synthetic resin lacquer. An example of such a protective coating is that of a mixture of a vinyl resin, an alkyl ester of acrylic or methacrylic acid, and an alkyl-aryl siloxane in solution in organic solvents, which has been specifically proposed for application to chromium plated automobile bumpers.
The recent trend toward lighter weight cars has increasingly directed attention to vehicle bumpers made by forming an aluminum alloy. In comparison with sheet steel bumpers, aluminum alloy bumpers and other automotive parts possess several advantages. These include lighter weight, with attendant saving in fuel consumption, reduced load on the vehicle suspension system, and the ability to eliminate expensive chromium plating. Aluminum alloy bumpers and other parts, whether anodized or otherwise protectively coated, do not corrode materially and do not lose their brightness, thus reducing the need for replacement or refinishing.
Steel vehicle bumpers are conventionally produced bu forming sheet steel of generally uniform thickness to a desired bumper shape by a series of press forming operations. Aluminum vehicle bumpers are produced by forming an extruded aluminum base alloy blank to provide a desired bumper shape. The extrudability of the aluminum bumper blank permits a wide range of profile configurations and designs, and the provision of additional thickness in those cross-sectional areas where it is needed for impact resistance and load capability, as well as increased dimensional stability. By providing areas of increased thickness in the cross-section of the aluminum extrusion, an aluminum bumper can be produced having increased impact and load capacities relative to a steel bumper of comparable size, while at the same time being much lighter in weight. The reduced bumper weight eases handling during manufacture and mounting operatons, and reduces the load on the suspension system of the vehicle, thereby offsetting the added weight of automotive accessories such as air conditioning units in the vehicles.
One mode of aluminum alloy bumper manufacture is to extrude a blank of a suitable alloy for working to produce the desired bumper cross-sectional contour, followed by quenching, and minor shaping operations such as cupping of the terminal ends of the blank. The bumper is then artificially aged, and may thereafter be chemically brightened and anodized to provide the final bumper. Such anodized aluminum alloy bumpers do not corrode readily and do not lose their brightness, thereby reducing the need for replacement or refinishing.
Anodized aluminum bumpers offer the advantage over chromium plated steel bumpers of elimination of the costly chromium plating operation. Moreover, while the underlying steel of a chromium plated steel bumper will be subject to rusting or corrosion if exposed by scratching or other disruption of the chromium plate, requiring replating, aluminum is far less susceptible to this type of damage.
However, anodizing has the drawback of requiring a large investment in equipment and input of constantly more expansive electrical energy. Anodized coatings also tend to limit the spectral reflectance of the anodized metal to 90% or less, usually about 85% to 88%, at least for alloys of adequate strength to be used for vehicle impact bumpers.
Accordingly, in the case of vehicle bumpers and other automotive trim made of aluminum base alloys, where high reflectance factor is a prerequisite, what the art has sought has been a practical and inexpensive method for providing on these articles a tough adherent coating, resistant to corrosive environments, but resulting in increased spectral reflectance.